NEEDLE LOSS AS A MECHANISMS OF WINTER DROUGHT AVOIDANCE IN BOREAL CONIFERS

Xylem (water) pressure potential was measured through one winter in evergreen black spruce (Picea mariana (Mill.) B.S.P.) and deciduous larch (Larix laricina (Du Roi) K. Koch) in Fairbanks, Alaska. Larch values averaged about - 1.0 MPa, with occasional dips to - 1.5 MPa. Black spruce showed similar values until May, when values dropped to - 2.5 MPa. Regression models indicate that desiccation of black spruce responds primarily to cumulative vapor pressure deficit (drought), which becomes severe as spring daylight rapidly increases (R2 = 80%). In larch, the effect of cumulative drought was offset by increased spring air temperatures.